This represents concatenation in regular expressions. There is no metacharacter for concatenation. It matches r1, and then r2 immediately following. r1 and r2 are regular expressions.

/CD/ will match any string CD where r1 matches C and r2 matches D.

/[x-z][1-3]/ will match any character x, y or z followed by any character 1, 2, or 3. Like x1, y3, z2, or x3. See [ and ] for information on character classes and how they participated in this regular expression.